1. Field of the Invention
The invention relates to a gradient coil system, in particular for a magnetic resonance apparatus, and a magnetic resonance apparatus having the gradient coil system.
2. Description of the Related Art
The magnetic resonance technique is a known technique for obtaining images of the interior of a body of an examination subject. For this purpose, a magnetic resonance apparatus includes a basic field magnet system and a gradient coil system. The basic field magnet system has, for example, a cylindrical hollow with the gradient coil system arranged within; this coil system is, in this example, likewise of a hollow cylindrical shape. A diameter of the hollow of the basic field magnet system, in particular in the case of superconducting basic field magnet systems, is proportional to its cost. The greater the diameter of the basic field magnet system to be designed, in order to accommodate an internal the gradient coil system, the more costly the basic field magnet system will be. Furthermore, the magnetic resonance apparatus comprises a radio frequency system which radiates radio frequency signals into the examination subject in order to trigger magnetic resonance signals and receives the generated magnetic resonance signals, which are used to create magnetic resonance images.
An imaging volume is defined inside a hollow of the hollow-cylinder-shaped gradient coil system. The basic field magnet system generates an (as far as possible) homogeneous, static basic magnetic field at least inside the imaging volume. The basic magnetic field is collinear with the principal axis of the hollow cylinder, to which a z-axis of a cartesian coordinate system is customarily assigned. The gradient coil system superimposes rapidly switched magnetic gradient fields on the basic magnetic field at least inside the imaging volume.
A gradient coil of the gradient coil system generates, for a particular direction in space, a gradient field which (in the ideal case) has exclusively a field component which is collinear with the basic magnetic field, at least inside the imaging volume. The field component has a predeterminable gradient which is, irrespective of location, approximately of the same magnitude at any point in time, at least inside the imaging volume. Since the gradient field is a time-variable magnetic field, the aforementioned is admittedly true of any point in time, but from one point in time to another point in time a strength of the gradient is variable. The direction of the gradient is generally fixed in a predetermined manner by the design of the gradient coil. To generate the gradient field, appropriate currents are set in the gradient coil. The amplitudes of the required currents are in the range of several hundred Amperes (A). The rates of rise and fall of the current (slew rate) amount to several hundred kA/s.
The gradient coil system is generally surrounded by conductive structures in which the switched gradient fields induce eddy currents. Examples of such conductive structures are a vacuum vessel and/or a cold shield of a superconducting basic field magnet system. The fields generated by the eddy currents are undesirable since, if no countermeasures are taken, they weaken the gradient field and distort it in its time characteristic. This impairs the quality of magnetic resonance images. These eddy current fields are compensated for, inter alia, by using actively screened gradient coils. A screening coil associated with a gradient coil generally has a smaller number of turns than the gradient coil, and is connected to the gradient coil in such a way that the screening coil has the same current flowing through it as the gradient coil, but in the opposite direction.
German patent document DE 197 22 211 A1 presents an example of a construction of a hollow-circular-cylinder-shaped gradient coil system having screening coils. The gradient coil system comprises, the following elements (from the inside outwards) which are formed in hollow-circular-cylinder-shaped regions arranged concentrically with one another: a first transverse gradient coil, comprising four saddle-shaped coil sections for generating a gradient field having a first gradient perpendicular to the principal axis of the hollow cylinder, a second transverse gradient coil, comprising four saddle-shaped coil sections for generating a gradient field having a second gradient perpendicular to the principal axis of the hollow cylinder, a first cooling arrangement, a longitudinal gradient coil for generating a gradient field having a gradient in the direction of the principal axis of the hollow cylinder, a shim assembly, a second cooling arrangement, a longitudinal screening coil assigned to the longitudinal gradient coil, a first transverse screening coil assigned to the first transverse gradient coil and a second transverse screening coil assigned to the second transverse gradient coil.
German patent document DE 196 53 449 A1 discloses a gradient coil system in which its gradient coils are arranged on two radially spaced surfaces lying symmetrical to a longitudinal axis of an examination chamber of a magnetic resonance apparatus, and the surfaces have, in radial section, a curvature such that a distance between the two surfaces is greater at the center than at the edge. The distance between the two surfaces gives rise to additional space laterally in the center of the examination chamber so that, for example, the diameter of the hollow of the basic field magnet system can be reduced.
German patent document DE 199 43 372 presents a hollow-cylinder-shaped gradient coil system of a magnetic resonance apparatus having an elliptical inner cross-section, a first and a second transverse gradient coil being arranged alongside each other on an inner cylinder envelope of the gradient coil system. The gradient coils comprise, in each case, four saddle-shaped coil sections arranged point-symmetrically with respect to a geometrical center point of the gradient coil system.
German patent document DE 196 12 478 C1 describes a hollow-circular-cylinder-shaped gradient coil system for a magnetic resonance apparatus having a first and a second transverse gradient coil of a segment type. The gradient coils comprise, in each case, a plurality of coil pairs which are arranged along a principal axis of the hollow cylinder and comprise, in each case, two circular-ring-segment-shaped coil sections arranged point-symmetrically with respect to the principal axis of the hollow cylinder. The coil sections of both gradient coils are arranged alongside each other on a circular-cylinder envelope of the gradient coil system; the two gradient coils have different numbers of coil pairs from each other and, when viewed in the direction of the principal axis of the hollow cylinder, the circular-ring-segment-shaped coil sections of the first gradient coil overlap those of the second.
Furthermore, German patent document DE 198 29 298 A1 presents an example of a gradient coil system which can be used to image, for example, only part of a patient""s body such as the patient""s head. This type of gradient coil system is often designed so that a linearity center of a gradient coil of the gradient coil system is offset relative to a geometrical center of the gradient coil system. Such an asymmetrical gradient coil system is often constructed with a torque-compensated conductor design. The conductor design is such that tilting moments, which occur with respect to a geometrical center point of the gradient coil system due to Lorentz forces when current flows in the gradient coil, cancel each other out. Each of the above-mentioned German patent documents is incorporated herein by reference.
An object of the invention is to provide an improved gradient coil system which, inter alia, allows a space-efficient construction of the gradient coil system.
The object is achieved by a gradient coil system, in particular for a magnetic resonance apparatus, that includes the following features:
the gradient coil system comprises a first and a second transverse gradient coil,
the gradient coils are arranged alongside each other axially offset on a common cylinder envelope and
the cylinder envelope has at least one circumferential line which is overlapped by both gradient coils.
The circumferential line corresponds to the circumference of a section which is taken perpendicularly to a generatrix of the cylinder envelope.
The following advantages are thus obtained for the inventive gradient coil system compared with a comparable conventional gradient coil system, in particular compared with a known gradient coil system in accordance with German patent document DE 197 22 211 A1: for an identical outside diameter, a greater inside diameter can be chosen, thereby resulting in greater patient comfort with regard to space. For an identical inside diameter, a smaller outside diameter can be chosen which also enables, for example, a corresponding reduction of a hollow of a superconducting basic field magnet system; this leads to a considerable cost saving. Furthermore, for identical dimensions, more room is obtained, for example, for arranging further coils and/or for designing at least one coil as a coil which is switchable and/or of a 3D design. Due to the arrangement of the two transverse gradient coils alongside each other, the first and the second transverse gradient coil are arranged in a manner such that they do not mutually overlap, thereby furthermore markedly reducing a risk of electrical flashovers between the transverse gradient coils. This also improves a partial-discharge behavior of the gradient coil system.
In an advantageous refinement, the gradient coil system comprises a cooling arrangement which is arranged immediately adjacent to the transverse gradient coils and between the transverse gradient coils and a further longitudinal gradient coil. Because the first and the second transverse gradient coil are arranged in a manner such that they do not mutually overlap, a large-area cooling of all three gradient coils is possible using the one cooling arrangement.
In an advantageous refinement, at least one of the transverse gradient coils is designed in a torque-compensated manner. This results in improved preconditions for reducing noise which may occur during operation of a gradient coil in a magnetic resonance apparatus.